1. Field of the Invention
The present invention relates to controlling devices for electric or fluid actuators, and to conveyances propelled by electric or fluid actuators. More particularly, the present invention relates to a controlling device that provides accuracy in the control of speed and direction of electrically or hydraulically propelled vehicles, that limits the rate of change in the difference of rotational speed of left and right propulsion motors, and that provides dynamic braking for electric motors.
2. Description of the Prior Art
Conveyances of various types, for transporting people, for material handling, and for propelling self-propelled machinery, have requirements for extremely high maneuverability.
One way to obtain extremely high maneuverability is to separately and variably control the speed and direction of rotation of left and right wheels or other traction elements. When the traction elements are moving at the same speed, but in opposite directions, the conveyance will pivot around in a fixed location, giving the ultimate in maneuverability.
In the design of some larger conveyances, such as bulldozers, it has been customary to use a system of clutches and brakes to control both the direction of rotation and the speed of endless tracks. While the use of clutches and brakes do provide extremely high maneuverability, including pivotal turns, the control of the direction of movement is far from being precise.
Systems using clutches and brakes for steering are not satisfactory for use with vehicles that must manuever in close quarters. For instance, small fork lift trucks that are used to unload boxcars and truck trailers need better control of maneuverability than can be attained by the use of clutches and brakes.
One way to attain more precise control of the speed and direction of the wheels, or other traction elements, is to convert the power into electric or hydraulic power, and then to utilize the controllability that is available with the use of electric or hydraulic power to separately and variably control the speed and direction of rotation of left and right propulsion motors.
However, achieving high maneuverability by separately controlling the velocity and direction of rotation of the wheels, or other traction elements, may make a conveyance difficult to control, or even dangerous.
For instance, it is desirable for some field harvesting machines, such as swathers, to have the ability to make pivotal turns; but it would be dangerous to attempt to make a pivotal turn at full harvesting speed.
If the rate of change of speed of the individual wheels is limited, then the machine will be sluggish in acceleration, and may be dangerously slow in deceleration.
The problem of controllability is particularly acute in wheelchairs or other conveyances which are steered by separately and variably controlling the speed of rotation and direction of rotation of the wheels.
Typically, a separate D.C. electric motor has been drivingly connected to left and right of the wheel of a wheelchair by chains or belts, and by friction rollers that separately engage the rubber tires of the wheels.
D.C. electric motors are capable of producing variable speeds that are in accordance with the electrical power that is delivered to them, and to the load that is imposed upon them. D.C. electric motors are also capable of reversible operation by reversing the electrical potential that is applied to the terminals.
Thus, manually actuated controls have been provided that separately and variably supply electric power from a battery to left and right propulsion motors to provide changes in speed, to provide turns, to reverse the direction of movement, and to make pivotal turns by rotating one wheel forward and the other wheel backward.
One popular type of manual control includes a control lever that is moved forward in accordance with a desired speed forward, that is moved rearward in accordance with a desired speed in reverse, that is moved both forward and to one side to make a turn while moving forward, that is moved directly to one side to make a pivotal turn.
However, many who are disabled have severe hand tremors that render them unable to use electrically propelled wheelchairs.
The hand tremors have not posed a serious problem for control of forward and reverse speeds, because hand tremors do not particularly effect the flexing action of the wrist that is used to control forward and reverse operation. Also, friction between the forearm and the armrest of the wheelchair helps to steady the arm.
But, those with severe hand tremors have been unable to control electrically propelled wheelchairs because hand tremors are primarily a torsional movement of the wrist, and the torsional tremor increases tremendously as an effort is made to position the control lever.
Thus, as a person with severe hand tremors has tried to control the positioning of the control lever, the tremor in his hand has moved the control lever rapidly from one side to the other, giving signals for first one, and then the other motor to rotate faster, resulting in rapid turns in one direction and then the other, and resulting in such erratic movement that he has not been able to control the wheelchair without bumping into other patients, furniture, doors, and walls.
Another problem that has attended prior art designs is that, even for those who do not have hand tremors, control of speed and direction has been uncertain because of the lack of dynamic braking. For instance, when the control lever has been positioned to reduce the electrical power to the left propulsion motor and thereby turn to the left, inertia of the wheelchair and occupant has driven the left propulsion motor through the drive train that connects the left propulsion motor to the left wheel; and the wheelchair has not responded by turning as signalled by the control lever.
A third problem has been a relatively poor overall efficiency of the drive trains that connect the electric motors to respective ones of the wheels; so that an unnecessarily large and heavy battery has been required.
Typical prior art designs have been so heavy and so unwieldy to transport that the usual way of transporting them has been to load them into a van by the use of a hydraulic lift. This has drastically reduced the mobility of the patient, has detrimentally reduced his opportunities to visit away from his home or the care facility, or has resulted in unnecessarily high expense for a vehicle that will accommodate both the patient and his wheelchair.
Even for the electric wheelchairs that have not been so unwieldy that they cannot be transported in a station wagon or in the trunk of a sedan, the excess weight of both the wheelchair and the battery have made it a strenuous job for friends or relatives to disassemble the wheelchair, load the wheelchair and battery separately into a car, reassemble the wheelchair at another location, and repeat the process when they return the patient to the care facility.
However, if prior art designs of electrically propelled wheelchairs had used drive trains with better efficiencies, particularly better efficiency when inertia of the conveyance and occupant is driving the electric motor, then the problem of insufficient dynamic braking, particularly in making turns, would have been more severe.
A fourth problem has been inadequacy, or the entire lack, of automatic dynamic braking in the power-off condition.
A fifth problem has been in poor contact life of the relays that are used to reverse the potentials of the electric motors, resulting in unnecessary expense, and resulting in loss of use of the wheelchair for extended periods of time while spare parts are being obtained and needed repairs are being made.
There are thousands of incapacitated people who would be able to gain a greater degree of self reliance, and some would be able to become a part of the work force of their country if they were able to control some type of self-propelled conveyance.
Thus the present invention can help handicapped people gain a better sense of dignity and self-worth, and to help many of them become productive members of society.